Ferritic stainless steel has a small coefficient of heat expansion, so has been used for automobile exhaust system parts subjected to repeated heating and cooling. Further, recently, due to its low susceptibility to stress corrosion cracks in case of deposition of snow melting agents in cold regions, it has started to be used for fuel inlet pipes of automobile fuel system parts. Most of these parts are thin welded pipes which are bent, bulged, expanded, etc. In particular, in fuel inlet pipes, Japanese Patent Publication (A) No. 2002-242779 discloses to secure a rust-prevention performance by integral and eccentric expansion of pipes. Sometimes the pipes are worked to two times or more the diameters of the original pipes. The pipes used therefore are required to have superior expandability.
In view of these requirements, as ferritic stainless steel pipe superior in workability, Japanese Patent Publication (A) No. 6-41689 defines Lankford values (r values) in the rolling direction of the material or the direction perpendicular to rolling, Japanese Patent Publication (A) No. 5-277769 discloses a method of production comprising preheating before laser welding or inside surface bead overlay, and then rolling of the weld bead to eliminate the overlay, while Japanese Patent Publication (A) No. 7-265941, Japanese Patent Publication (A) No. 7-266072, Japanese Patent Publication (A) No. 7-286239, Japanese Patent Publication (A) No. 8-257777, and Japanese Patent Publication (A) No. 2000-326079 disclose combining roll-less pipemaking and laser welding during which defining the heat input of the laser welding machine and pipemaking speed, the relationship between the bending roll radius and plate thickness in the roll-less pipemaking method, and the sizing conditions to adjust the hardness difference between the matrix and the weld zone or adjust the amounts of C, N, and O and inclusions in the laser weld zone and thereby improve the bulgeability, expandability, and bendability of the weld zone, but with just adjusting the grade of the material, the shape of the weld bead, or the hardness of the weld bead, it is difficult to withstand extremely severe expansion to two times or more the diameter of the original pipe and accompanied with eccentric expansion.
On the other hand, in terms of the material as well, the usage environment has become harsher and improvement of the lifetime and other facets of durability has been demanded, so improvement in the corrosion resistance, high temperature salt damage resistance, oxidation resistance, high temperature strength, and heat fatigue has been sought. Cr, Mo, Nb, and other alloying elements are being included in increasing amounts as a general trend. This is a direction not preferable from the viewpoint of the workability, so a welded pipe material achieving both usage performance and workability is being sought.